This invention relates to adding and dropping of selected wavelength components in wavelength division multiplexing (WDM).
Fiber optic telecommunication networks are increasingly used for data transmission because of the high data transfer rates available with such networks. A plurality of substantially different wavelength components can be transmitted along a single optical fiber, and these wavelength components can be combined and transmitted as a single signal. Each wavelength is referred to as a xe2x80x9cchannel.xe2x80x9d A crucial feature is exchange of wavelength components between signals propagating on xe2x80x9cloopsxe2x80x9d within a network. This exchange occurs at connector points within a loop, or at points where two or more loops intersect.
Exchange of data signals involves exchange of matching wavelength components associated with two different loops within a network. In this exchange, a signal traveling on a first loop on the network drops a first wavelength component, which is picked up by a second loop, and simultaneously picks up a corresponding wavelength component that has been dropped by the second loop. Addition of a selected wavelength component and dropping of this wavelength component always occurs together, between first and second loops.
Conventional implementation of add/drop actions in a fiber optic WDM system often require use of more wavelength discrimination hardware than should be needed for such purposes. Each add/drop combination requires use of an add module and of a separate drop module. For example, a conventional 1xc3x972 or 2xc3x971 optical add/drop multiplexer (OADM) allows only addition of a wavelength component or dropping of a wavelength component, but not both at the same time, analogous to simplex operation of a transmission line in which a signal may be transmitted in only one direction at a time.
For a wavelength sequence, such as xcex1, xcex2, . . . , xcex40, a single OADM device has an associated optical loss of about 0.4 dB for reflection and an associated optical loss of about 1 dB for one-way transmission. To perform an add operation and a drop operation for a pair of wavelength components using a conventional OADM device, an add module and a separate drop module are required, with an associated insertion losses of about 0.8 dB for two reflections. This loss might be reduced if a (non-conventional) OADM device can be provided that performs wavelength addition and wavelength dropping in a single device.
What is needed is a simpler OADM system that allows simultaneous addition and dropping of M wavelength pairs without requiring use of 2M OADM modules and related coupling for this purpose. Preferably, such a system should allow use of one pair of coupling channels in a single OADM device to carry one pair of wavelength components to be used in the add/drop operation.
These needs are met by the invention, which provides a simpler and more reliable system for optical add/drop multiplexing in a fiber optic. Where a conventional OADM approach would require two OADM modules, for example, to provide both add and drop capability for a single wavelength component (2xc3x972 OADM), the invention provides this capability using one such OADM module, differently configured. More generally, M OADM modules, suitably connected in tandem, provide an (M+1)xc3x97(M+1) OADM system for control of M wavelength pairs, with M=1, 2, . . . An OADM for a single wavelength uses a dual fiber collimator at a first end and at a second end to Each OADM module component has an associated wavelength filter that transmits a selected wavelength, xcex=xcex2, and reflects each of a sequence of wavelengths, xcex1, xcex3, xcex4, xcex5, . . . , xcex40, from an original sequence xcex1, xcex2, xcex3, xcex4, xcex5, . . . , xcex40. If a light beam containing the wavelength xcex=xcex2 approaches the filter in an OADM module component, the wavelength xcex=xcex2 is transmitted at the filter, and thus dropped, and other wavelengths from the original sequence, namely xcex1, xcex3, xcex4, xcex5, . . . , xcex40, are reflected at the filter. Alternatively, if a light beam containing the wavelengths xcex1, xcex3, xcex4, xcex5, . . . , xcex40 approaches the filter from a first direction (e.g., from the left) and a light beam having the wavelength xcex=xcex2 approaches the filter from the opposite direction (e.g., from the right), the wavelength xcex=xcex2 will be transmitted through the filter and will join and augment the original light beam to form an exiting light beam that contains the wavelengths xcex1, xcex2, xcex3, xcex4, xcex5, . . . , xcex40.